This invention relates generally to the field of medical devices, and more particularly to systems, devices, methods and applications for measuring current drain or charge depletion for medical devices.
Medical devices, including cardiac stimulus devices such as implantable cardiac pacemakers and implantable cardioverter defibrillators (ICDs), are surgically implanted within a patient. Cardiac stimulus devices have one or more electrical leads with one or more electrodes that conduct signals to and receive signals from the patient""s heart. As such, cardiac stimulus devices provide electrical therapy to the patient. A programming device or programmer is able to communicate with the medical device through a communication link. The communication link provides means for commands and data to be transmitted and received between the programmer and the device.
Longevity is an important aspect regarding the performance of the medical device, and it is determined primarily by the capacity of the battery and the current drain of the electronics; i.e. battery status. The current drain depends on a number of variables, including the impedance(s) of the pacing lead(s), the ratio of sensed beats to paced beats, the background current of the device""s electronics, and the characteristics of the pacing pulse(s) such as pacing rate, pacing mode, amplitude and width. Approximations for current drain and/or charge depletion for the medical device are obtained by estimating these variables. However, estimating these variables is difficult because they vary over the lifetime of the device. Tradeoffs that sacrifice the longevity of the medical device for the safety of the patient may be required to maintain an appropriate safety margin that accounts for the granularity and uncertainty in estimating these variables.
Latent electrical faults within the implanted device may cause elevated current drains. These electrical faults can go undetected early on in the life of an implanted device without a means for measuring and reporting the current of the implanted device. A problem with reporting current measurements for an implanted device is that the communication system, such as a telemetry circuit, draws considerably more current during a communication session than in a quiescent period. This extra current drawn by the telemetry circuit can adversely affect the current measurements.
Therefore, there is a need in the art to provide a system and method for measuring the current drain or charge depletion for implantable medical devices, and to provide applications for determining the battery status, detecting electrical faults, and reporting the battery current.
The present subject matter addresses the aforementioned problems by providing systems, devices and methods for measuring current drain and tracking the charge depletion. Applications include a method for determining battery status, for determining potential faults, and for displaying or reporting current drain.
One aspect provides a medical device, comprising a battery, a pulse generator, an electrode system, and a current measuring device. The pulse generator draws a pulse generator current from the battery. The electrode system is in electrical communication with the pulse generator and applies electrical therapy, such as pacing pulses, to a patient. The current measuring device determines the pulse generator current, and is capable of being calibrated as it is determining the pulse generator current. Therefore, the current measuring device is able to continuously measure or track the current drain from the battery and still be calibrated or periodically calibrated to maintain the accuracy of these current measurements.
One embodiment of the current measuring device comprises an oscillator and a counter. The oscillator produces an oscillating output that has a frequency of oscillation dependent on the pulse generator current, and the counter provides an oscillation count for the oscillating output, i.e. the counter counts the number of oscillations in the oscillation output over a known time interval. The pulse generator current is determined from the oscillation count.
One embodiment of the current measurement device comprises a current source that provides an oscillator input current that is dependent on the pulse generator current to the oscillator. One current source embodiment includes a current divider for dividing current drawn from the battery into the pulse generator current and an attenuated current. The current divider includes a sense resistor, an attenuation resistor, and an operational amplifier. The pulse generator current flows through the sense resistor, and the attenuated current flows through the attenuation resistor. The operational amplifier applies negative feedback to provide a voltage drop across the attenuation resistor that is substantially equal to the voltage across the sense resistor.
In another embodiment, the device also includes a second current source having a second operational amplifier. Both operational amplifiers have an autozeroing feature that compensates for an offset voltage that could adversely affect the value of the attenuated current, and thus adversely affect the determination of the pulse generator current. The pulse generator current is continuously measured using one of the two current sources. As one current source autozeros, the other current source continues to operate.
Another aspect provides a method for determining current drain for a medical device, wherein the oscillations are counted for a period of time, and the pulse generator current is determined by the oscillation count over the period of time. Another aspect provides a method for tracking charge depletion of a battery in an implantable medical device, wherein the charge depletion of a battery is determined by continuously summing the oscillation count over successive periods of time.
These and other aspects, features, embodiments, applications, and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.